In order to use thin wall, monowall, or multiple layer wall plastic tubing (i.e., wherein the ratio of OD: total or cumulative Wall Thickness&gt;4:1) in an automobile fluid handling system, it generally becomes necessary to bend and shape such tubing to fit the application. Such tubing may be of monowall thermoplastic construction, or multiple (e.g., two through seven) layer tubing wherein all layers are of thermoplastic material or wherein some layers are of metallic material and the remainder of plastic material. As with any thermoplastic tube, the addition of heat is necessary to not only allow the tube to bend to the required shape but also to set the tube in this shape so that it remains in the bent shape after the bending is completed. This bend forming process has proved to be a difficult task because of the nature of the plastic tube to kink or deform at the bend, which at the minimum can cause unwanted turbulence of liquid passing through the tube. Such deformations also cause weaknesses in the tube which would not be acceptable for the end use from a performance standpoint.
Bend forming the tube only in the required areas requires a knowledge of how the bending stress is distributed in a tube as it is being bent. This stress can be simplified for this explanation to be shown as axial stresses acting along a given zone of the walls of the tube in either a compressive or tensile state. Tensile stresses acting in the walls of the tube stretch the tube so as to lengthen it, and compressive stresses compress the walls of the tube so as to shorten it. Most of the deformation problems associated with bending the tube occur because of the compressive stresses that occur on the inside of the bend radius. As the tube is bent around a bending die, material in the tube must flow either in a compressive or tensile state to conform with the physical bending forces that are being exerted on the tube. This material flow is what allows the tube to eventually conform to the shape of the bending die. The tube bending stresses resolve into compressive stresses acting on the inside radius of the tube during bending that make the material on the inside radius of the bend want to push together. However, this is physically impossible because the material will not naturally form a thicker wall while maintaining the shape of the original tube. It will instead flow in what is referred to as "kinks" or unwanted deformation, actually pushing or folding the tube wall material up and down away from the bend die.